This application claims the priority of Japanese Patent Application No. 27659/1995 filed Jan. 23, 1995, which is incorporated herein by reference.
InP type photodetectors for the long wavelength light, in general, have been produced by the following method. An InP wafer is prepared as a substrate crystal. An n-type InP buffer layer, an n-type InGaAs receiving layer and an n-type InP window layer are epitaxially grown on the n-type substrate. Then, an insulator layer, e.g., SixNy, is deposited as a mask on the window layer. Windows are perforated through the mask to the epitaxial layers by photolithography. P-type impurity, e.g., zinc (Zn), is thermally diffused through the opening on the InP window layer and the InGaAs receiving layer. The p-type impurity-diffused parts of the window layer and the InGaAs become a p-type region. A ring-shaped p-side electrode is formed on the p-type region. The opening encircled by the p-side electrode becomes an area which receives light beams. An n-side electrode is formed on the n-type InP substrate. An antireflection film is deposited on the light receiving area for heightening the sensitivity. These processes are called "wafer process". Then, the wafer is sliced lengthwise and crosswise along cleavage lines into a plurality of individual photodiode chips.
The wafer having an n-type InP substrate, an n-InP buffer layer, an n-InGaAs receiving layer and an n-InP window layer before the formation of electrodes is sometimes called an "epitaxial wafer" or an "epitaxial crystal". The region which has been doped with the p-type impurity is named a p-region. The p-region has a dish-like section. The doping of the p-type impurity converts a part of n-type layer into p-type region. The boundary between the p-region and the n-region is called a pn-junction.
The InGaAs light receiving layer has a smaller band gap Eg 1 (width of the forbidden band) than InP whose band gap is denoted by Eg2. Namely, Eg1&lt;Eg2. A semiconductor absorbs photons (quanta of light), when the semiconductor has a smaller band gap Eg than the photon energy h .nu.. If light having energy bigger than Eg1 but smaller than Eg2 enters the above InGaAs photodiode (Eg1&lt;h .nu.&lt;Eg2), the InP (Eg2) is transparent to the light (h .nu.) but the InGaAs (Eg1) layer absorbs and detects the light.
Therefore, the InP layer above the InGaAs receiving layer (absorbing layer) acts as an window which does not hinder the light from penetrating into the inner layer without loss.
The electrode which is formed in an ohmic contact on the p-type region is a ring-shaped electrode. Then, light enters the central part enclosed by the ring electrode. Thus, the electrode on the p-region is called an "annular electrode", or a "ring electrode". The same electrode is often called a "p-side electrode" or "p-electrode".
A flat wide n-electrode is formed on the bottom side of the n-InP substrate. If the substrate is semi-insulating, an n-electrode is sometimes formed partially on an n-region of an epitaxial crystal. Both the p-electrode and the n-electrode are ohmic electrodes which have an ohmic contact with the underlying layer or substrate.
The antireflection film is made from SixNy (which is often represented in brief as SiN by omitting suffixes x and y) or SiO.sub.2 or other transparent dielectrics. The antireflection film which scarcely reflects incident light can be fabricated by selecting pertinent reflection ratio of the film. The part outside of the annular electrode is also covered with a dielectric film.
Such a structure is generally employed for making photodiodes. In use, the n-electrode and the p-electrode are reversely biased. A depletion layer is yielded on the pn-junction. The applied bias generates an electric field which is directed from the n-type region to the p-type region. The light receiving region enclosed by the annular p-electrode is irradiated by light beams which have emanated from an optical fiber and have converged on the photodiode via a lens. The light beams pass through the window layer without loss and arrive at the InGaAs receiving layer. The light generates pairs of electrons and holes. Electrons make their way to the n-electrode and holes progress to the p-electrode. The flow of the electrons and the holes is called a photocurrent. The photocurrent is in proportion to the power of the incident light.
For example, Japanese Patent Laying Open No.4-111477 (111477/'92) describes a method of producing such a photodiode. The proposed photodiode has a wide p-region which extends beyond the annular p-electrode for suppressing stray light reaching outside of the annular electrode from yielding a retarded photocurrent. In the device, the holes which have been generated outside of the p-electrode cannot cross over the pn-junction and cannot reach the p-electrode. Thus, no retarded photocurrent is induced.
Another important problem is the coating of the peripheral part outside of the annular electrode. The peripheral part of the window layer is coated with an insulating layer. The insulating layer has the role of protecting the InP crystal from chemical reaction and contamination. The insulating layer is called a "passivation film". SixNy, SiOx or other dielectrics are employed for making passivation films. SiN (suffixes x and y are omitted), in particular, is excellent in cohesion to InP crystals.
It is undesirable for light beams to enter the peripheral region outside of the annular electrode. For example, Japanese Patent Laying Open No.64-23580 provides an InGaAs absorbing layer around the p-annular electrode of a photodiode. The peripheral InGaAs layer absorbs the light which arrives at the periphery. Thus, no light enters into the outside region of the photodiode.
Shortening the time of response requires low resistance at electrodes. The resistance between the electrode and the semiconductor is called an "electrode resistance" or a "contact resistance". There is no problem for the n-electrode which is formed on the n-region, since the contact resistance is low enough because of the wide contact area and the high impurity concentration at the n-electrode.
A problem accompanies the p-side electrode, since a narrow contact area has a tendency of raising the electrode resistance. When a p-electrode is formed directly on the p-region, the resistance cannot be reduced below a certain value, no matter what low-resistant material is utilized to build p-electrodes. Fabrication of lower contact resistant p-electrodes requires some contrivance other than the selection of the electrode material.
Japanese Patent Laying Open No. 62-62566 proposes an improvement of forming an undercoating layer of InGaAs for reducing the contact resistance of p-electrodes. The contact resistance should be decreased by forming an InGaAs undercoating layer on the p-InP layer and depositing a p-electrode on the undercoating layer. Namely, No.62-62566 makes a p-electrode of Ti/Pt/Au or Cr/Au on an InGaAs undercoating layer which has been deposited on the p-InP layer.
Why is InGaAs pertinent to the undercoating for ohmic p-electrodes? The difference of work functions between InGaAs and the p-electrode metal is smaller than the difference between InP and the electrode metal. A smaller difference of work functions ensures the electrodes a lower contact resistance. This is one reason for the excellence of InGaAs as an undercoating material. InGaAs crystals can contain higher density of impurity than InP crystals at the interfaces between a metal and a semiconductor. The high density of impurity enables p-electrodes to reduce the contact resistance.
The InGaAs of the light receiving layer must not be confused with the InGaAs of the undercoating. The InGaAs receiving layer has been formed in an epitaxial wafer. An electric field can be built in the InGaAs receiving layer for inducing a photocurrent by an incidence of photons, since the InGaAs layer has a small density of impurity and a high resistance. The InGaAs layer makes better use of the narrower band gap than that of InP for sensing photons.
On the other hand the undercoat InGaAs layer is highly doped with impurity. The undercoat InGaAs takes advantage of the low difference of the work functions between the metal and InGaAs, the high carrier density and the low resistance for leading currents.
A new problem arises from the use of the same material for accomplishing these two different objects. The light which should be detected by the InGaAs photodetector has a energy h .nu. which is smaller than the InP band gap Eg2 but bigger than the InGaAs band gap Eg1. Namely, Eg1&lt;h .nu.&lt;Eg2. InGaAs is not transparent to the light, because Eg1&lt;h .nu.. InGaAs can absorb the light. If the undercoat InGaAs layer extends beyond the p-electrode, the extending parts will partially shield the receiving area from the incidence of the light. The existence of the projecting parts is undesirable, since the projecting parts absorb a part of the entering light and attenuate the power. Thus, it is preferable to equalize the breadth of the undercoat InGaAs to the breadth of the p-electrode and to suppress the under coat InGaAs layer from expanding out from the beneath the p-electrode. But it is very difficult.